1. Field of the Invention
The present invention relates to a carbon dioxide sensor used for an indoor and outdoor environmental monitoring and control, an agro-industrial process such as greenhouse horticulture, disaster prevention, a measurement of metabolic function of body surface, and a medical care.
2. Description of the Related Art
In recent years, a need for carbon dioxide sensor is increasing particularly in detection of dirty air in the room caused by the spread of air conditioner, detection of contamination air in livestock facilities, control of the growth of plants in greenhouse and various industrial processes, and various kinds of carbon dioxide sensors have been reported.
Specifically, an infrared absorption carbon dioxide sensor has been in practical use, for example. However, this carbon dioxide sensor is not widely used because of the size and the cost of the apparatus. In addition, a carbon dioxide sensor using a semiconductor has been also in practical use; however, it is difficult to monitor only the carbon dioxide concentration due to poor carbon dioxide selectivity.
On the other hand, a carbon dioxide sensor using a solid electrolyte has been proposed as a compact and inexpensive sensor. An example of such carbon dioxide sensor is so-called an electromotive force detection sensor obtained by forming a pair of electrodes on a solid electrolyte having an alkali metal ion conductivity such as NASICON (sodium super ion conductor: Na3Zr2Si2PO12). One electrode is used as a detection electrode with a metal carbonate layer such as sodium carbonate which forms a dissociation equilibrium with carbon dioxide. The other electrode is used as a carbon dioxide non-sensitive electrode (refer to Maruyama et al. “Solid State Ionics”, Vol 23, No. 1/2, p.107–112 (1987)).
According to this carbon dioxide sensor, an excellent carbon dioxide sensing property can be achieved. However, there has been a problem that this carbon dioxide sensor is susceptible to humidity, because the metal carbonate is used as the detection electrode. Therefore, a carbon dioxide sensor that is lightly affected by humidity by utilizing the mixture of alkali metal carbonate and alkali earth metal carbonate, or solid solution for the detection electrode has been proposed (refer to Publication of Examined Application No. Hei 7-85071, Japanese Patent No. 2598172, Japanese Patent No. 2974088 and Japanese Patent No. 2974090, for example).
The operation temperature of the carbon dioxide sensor is as high as 400° C. to 700° C., so the overall power consumption of the sensor is high and there has been a problem that the heat deterioration of materials occur. Further, heat of a few hundreds of temperature raises temperature around the sensor even from a small heater and generates convection of the air. This affects subtly on the environment for monitoring, which is a problem.
Accordingly, a carbon dioxide sensor using a tin oxide (SnO2) semiconductor containing stibium (Sb) or vanadium (V) as the detection electrode has been proposed (refer to S. Breikhin et al. “Applied Physics”, A57, 37–43 (1993), for example). Another carbon dioxide sensor in which the detection electrode is formed of a metal oxide layer, or a metal oxide layer dispersed metal carbonate or hydrogen carbonate has been proposed (refer to Japanese Patent Laid-Open No. Hei 11-271270 or Japanese Patent Laid-Open No. 2000-88799). These carbon dioxide sensors are operatable at room temperature.
However, the carbon dioxide sensor using the tin oxide semiconductor as the detection electrode requires four or more minutes for response and the difficulty of prompt measurement has been a problem. In addition, there has been another problem that the response time or sensitivity varies by humidity because water vapor is involved in the detection mechanism in this carbon dioxide sensor. Also, the carbon dioxide sensor using the metal oxide is susceptible to humidity. Although the influence of humidity on the sensitivity is small, the absolute value of the electromotive force output varies and the correction is required according to humidity, which is a problem.